Known appliances of this type have the general appearance of an improved calculator, being provided with an alphanumeric keyboard, and a small loudspeaker enabling dual-tone multifrequency (DTMF) dialing signals to be emitted corresponding to the various digits of a telephone number.
Ergonomically, such appliances turn out to be rather unsatisfactory, insofar as enriching and updating the directory and searching for a telephone number are operations that are lengthy and tedious: a person's name or part of the name must be keyed in via the alphanumeric keyboard which has a very large number of keys occupying a very small space, and it is necessary to apply procedures that occasional users find relatively difficult to remember.
In addition, such appliances are practically impossible to use with one hand only even though the other hand is holding the telephone handset, etc. This difficult and fiddly handling and also the rather long time taken to find and dial a number explain why such appliances have been moderately successful only, and why users often revert after a while to a conventional paper address book.